The invention relates generally to thyristors, and more particularly to thyristors of the type having emitter and base zones extending to a top surface on which cathode and gate electrodes are formed.
Separate masking steps are generally employed in forming the emitter zone and the electrode metalization areas on the top surface of a thyristor device. The thyristor emitter covers a portion of the top surface of a semiconductor chip, with the emitter-based junction extending to the top surface and defining a turn-on line. The position of the gate metalization with respect to the turn-on line can significantly affect the turn-on potential along the emitter-base junction. To produce thyristors with high di/dt ratings it is necessary to provide both a long turn-on line and relatively even turn-on potential along its length during triggering. The separate masking steps employed in forming the emitter zone and gate metalization makes precise gate alignment difficult. If the masks are not precisely aligned the metalization will be at unequal distances from the turn-on line. The result is nonuniform turn-on with a resultant lower di/dt rating.
One prior art technique for correcting emitter-gate misalignment is to etch the emitter zone to a precise distance from the edge of the gate metalization. Such a technique cannot alter the position of emitter shorts located at fixed positions within the emitter zone. Such emitter shorts join the thyristor base zone to the cathode metalization at discrete locations and play an important part in determining the potential along the turn-on line. Unless the gate metalization is a substantially uniform distance from the closest emitter shorts, uneven turn-on line potential will result even if an alignment etch is used.